state of the art in tunnel monitoring - ytmkytmk.org.tr/files/files/09_tug_monitoring.pdfaustrian...

Austrian Tunnelling Seminar Ankara, March 31st & April 1st, 2015

STATE OF THE ART IN TUNNEL MONITORING

Wulf Schubert

Graz University of Technology3G-Gruppe Geotechnik Graz ZT GmbH

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Monitoring


■ Uncertainties in the geological model ■ Uncertainties and spread of ground parameters■ Simplifications in the mathematical models used for design

inaccurate design and thus residual risk during construction

WHY MONITORING ?

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Monitoring


WHY MONITORING ?

■ Observations and measurements are used to □ Assess the stabilty of the underground structure□ Verify/falsify assumptions made during the design□ Adjust excavation and support methods □ Improve the ground model□ Detect important features outside of the visible area□ Quality control and conservation of evidence□ Basis for back analyses

■ In addition measurements are an important basis for the geotechnical safety management

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Monitoring


REQUIREMENTS FOR SUCCESSFUL APPLICATION OF OBSERVATIONAL APPROACH

■ Expected behaviors and acceptable limits must be defined prior to construction

■ Instrumentation, monitoring layout, and reading frequency must be in a way to allow capturing expected behaviors

■ Analysis of results must be sufficiently rapid in relation to the possible evolution of the system

■ Appropriate site organization to allow for a short response time in case actual behavior deviates from the predicted/acceptable

■ Safety management plan including contingency measures for cases, where actual behavior deviates from expected

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Monitoring


REQUIREMENTS FOR MEASUREMENT DATA COLLECTION

■ Take readings as soon as possible■ Make sure face position is recorded correctly■ Process data as quickly as possible in high quality■ Comment on data quality if required■ Be aware that accuracy and quality of data is important for the

decision making■ Protect measurement devices and targets against damage

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MONITORED PARAMETERS

■ Absolute displacements in space■ Relative displacements

□ Tape measurements□ Extensometer□ Measuring anchor

■ Surface settlements□ Levelling□ Horizontal inclinometers

■ Inclinations□ Inclinometers□ Tilt meters

■ Strains (strain gauge)

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METHODS OF DISPLACEMENT MONITORING

■ Relative displacement measurements□ Relatively high (sometimes only

apparent) accuracy□ Only information on elongation or

shortening of measured length□ Physilcal access to measurement pins

required□ Hindrance of works by platform and

tape across the tunnel

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METHODS OF DISPLACEMENT MONITORING

■ Absolute displacements □ Free positioning, thus minimal

interference with operation□ No physical contact to targets

required□ Measurement of spatial movements□ Accuracy influenced by unfavourable

posiion of instrument, refraction, dust, vibrations, stability of „fixed“ points

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STATE OF THE ART

■ Measurement of absolute displacements has in many countries replaced the convergence measurements

■ The information quality and quantity is much higher than with the traditional methods

■ The knowledge of the spatial position of each point at any time has opened a wide field for valuable evaluation methods

■ Additional instrumentation (like extensometers) only required in special situations

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RESULT OF TRADITIONAL CONVERGENCE MEASUREMENT

■ Display of relative displacements does not allow properly identifying anisotropic displacements

-90

-80

-70

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-20

-10

06.7 11.7 16.7 21.7 26.7 31.7

Time

Con

verg

ence

(mm

)

H1

DL DR

H1

DR

DL

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SAME DATA WITH ABSOLUTE DISPLACEMENT MONITORING

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TYPICAL INSTRUMENTATION FOR SELECTED SECTIONS

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DATA EVALUATION

■ Traditionally displacements are plotted versus time, and the results visually inspected. This makes interpretation of readings difficult in case of an unsteady andvance rate

■ It is advisable to plot the displacements against face advance, as this is the most prominent influencing factor, or use a mathematical model, which considers time and advance effects (for example software GeoFit)

■ Measurement of spatial displacements has opened a wide field of different assessment methods, like:□ Spatial orientation of displacement vectors for identification of

geological features outside the visible area□ Assessment of lining loads

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TYPICAL DEVELOPMENT OF DISPLACEMENTS

-25 -20 -15 -10 -5 0 5 10 15 20 25

time / distance

disp

lace

men

t

displacement ahead of face

displacement behind face

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POTENTIAL EVALUATION METHODS

■ Displacements versus time or face distance□ Can give good overview on stabilization process□ But only one component plotted, thus evaluation cumbersome

■ Deflection lines□ Give good overview of one componenet over longer tunnel section,

but also only one component plotted■ Displacement vectors

□ Nicely show influence of ground structure, system response, anisotropic displacements; usually only one section at a time can be shown

□ Spatial displacement vector orientation changes can show changing ground conditions ahead of face

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time

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00 5 10 15 20

disp

lace

men

ts

time

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00 5 10 15 20

disp

lace

men

tsDISPLACEMENTS versus TIME

■ With the d/t diagram, apparently larger displacements are interpreted initially. In particular if one estimates final displacements from first day(s) displacements, result may be misleading

2m/day

5m/day

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DISPLACEMENTS versus FACE DISTANCE

■ With the d/x diagram the difference is marginal, provided that time dependent displacements are not dominant

face distance

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00 10 20 30 40 50

disp

lace

men

ts

face distance

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00 10 20 30 40 50

disp

lace

men

ts

2m/day

5m/day

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UNSTEADY ADVANCE RATE – DIFFERENT PLOTS

■ Displacement – distance diagram not influenced by unsteady advance rate, thus easy to interprete (decreasing displacement rates with increasing distance)

time

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00 5 10 15 20

disp

lace

men

ts

face distance

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00 10 20 30 40 50

disp

lace

men

ts

1m/day

4m/day

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MATHEMATICAL DESCRIPTION OF DISPLACEMENT DEVELOPMENT

■ Sulem and Panet have developed an empirical relationship for the displacements behind the face in relation to face advance and time

■ General form of function:

)(1 xC

Panet, M., Guenot, A.: Analysis of convergence behind the face of a tunnel, Tunnelling 1982, The Institution of Mining and Metallurgy, 197 – 204Sulem J., Panet M., Guenot, A.: Closure Analysis in Deep Tunnels, Int. Journal of Rock Mechanics and Mining Science, 24, 1987, pp 145 – 154, Pergamon Press

)(**)(, 21 tCACxCt)C(x x

Advance dependent component

)(2 tC Time dependent component

xC Final time independent displacementA Final time dependent displacement

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MATHEMATICAL DESCRIPTION OF DISPLACEMENTDEVELOPMENT

■ The advance dependent function reads as:

■ The time dependent component reads as:

n

TtTtC 1)(2

2

1 1)(xX

XxC X … shape parameterx … distance between face and observed section

T … shape parametert … elapsed time between excavation and observation

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MATHEMATICAL DESCRIPTION OF DISPLACEMENTS

■ Using

and for n=0.3, the function for the development of displacements reads as follows:

xCAm

3,02

111,tT

TmxX

XCtxC x

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DISCUSSION OF PARAMETER X

■ X describes the shape of the advance dependent functionthe smaller X is, the shorter is the length influenced by the excavation

■ The value of X depends on ground utilization and the ground structure; the higher the ground is stressed, the higher is the value of X. Panet proposed to use for X = 0.84 * plastic radius(See also Pilgerstorfer, 2009 and Hoek, 2008)Also the relative orientation between tunnel axis and foliation influences the vale X Low values for strike perpendicular to axis, high values for parallel strike

Range of X for dia 10m tunnels: 4 to 30, usually around 8 to 15

Pilgerstorfer, T. & Schubert, W. 2009. Forward prediction of spatial displacement development. In Ivan Vrkljan, Rock Engineering in Difficult Ground ConditionsSoft Rocks and Karst, 495-500. London: Taylor \& Francis Group.Hoek, E., Carranza-Torres, C., Diederichs, M.S. and Corkum, B. 2008.Integration of geotechnical and structural design in tunnelling.Proceedings University of Minnesota 56th Annual Geotechnical Engineering Conference. Minneapolis, 29 February 2008, 1-53

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EXAMPLE X = 6/25; m= 0; C=-70

Distance to face

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00 20 40 60 80 100 120 140

disp

lace

men

ts

Distance to face

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00 20 40 60 80 100 120 140

disp

lace

men

ts

X= 25

X= 6

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DISCUSSION OF PARAMETERS m AND T

■ m describes the proportion of the time dependent displacements

Example:time indep. displacement Cx∞=100, m=0.3C(x,t) for infinite x and infinite t the value of 130)

■ The parameter T, like the parameter X describes the shape of the function (time dependent displacements finished early or last over longer period of time).

■ Values of m and T very much depend on time dependent characteristics of the ground and the stress levelCommon values for m and T from fitted measured displacements: m=0.1-0.8T=0.5-2

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Distance to face

-100-90-80-70-60-50-40-30-20-10

00 20 40 60 80 100 120 140

disp

lace

men

tsExample for X = 25; m= 0.3; T=0.5; C=-70

Distance to face

-100-90-80-70-60-50-40-30-20-10

00 20 40 60 80 100 120 140

disp

lace

men

ts

m=0.3

m=0

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PREDICTION OF DISPLACEMENTS TOP HEADINGwith Software GeoFit

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EFFECT OF TOP HEADING INVERT

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PREDICTION FOR BENCH AND INVERT EXCAVATION

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COMPARISON PREDICTION - MEASURED

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DEVIATION FROM „NORMAL“

Failure top heading invert

collapse

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POTENTIAL EVALUATION METHODS

■ Trends□ Information compressed□ Various trends can be shown (displacements, ratios, orientations,

etc)

■ Contour plots□ Presently mainly used for surface settlements and lining utilization

■ Lining utilization□ By recalculating strains from measured displacements, and using an

appropriate material model, stresses in the lining can be evaluated

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DEFLECTION LINES

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HOMOGENOUS GROUND

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INCREASED DISPLACEMENTS NEAR FACE

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ASSUMPTION: FAULT AHEAD FACE

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FURTHER DEVELOPMENT

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TREND LINE

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DISPLACEMENT VECTORS - GENERAL

■ Measurement of absolute displacements allows to observe the movement of each point in space

■ This is very valuable for assessing the influence of ground structure and quality on the system behaviour

■ Processes occuring outside the visible area show in the displacements, and can be realtively easily interpreted

■ Method less suitable for direct observation of normal stabilization process

■ Monitoring targets usually mounted on lining, thus result of mesasurements not always reflects ground behaviour(slip between lining and ground)

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PLOT IN CROSS- AND LONGITUDINAL SECTION

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STRONGLY ANISOTROPIC BEHAVIOUR

■ Features outside the excavation area (faults, slickensides,…) change the stress situation and/or kinematics, and thus the effects can be seen in the displacement characteristics

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INFLUENCE OF FOLIATION ON „NORMAL“ ORIENTATION

Excavation direction Excavation direction

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1251

1251

INFLUENCE OF SINGULARITIES

■ faults right and left of tunnel, strong influence on left side

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■ reduced influence on left side, increased influence on right side

1275

1275

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1289

1289


■ fault in face, low influence

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DISPALCEMENT VECTOR ORIENTATION

■ When relatively soft rock mass is ahead of the face, change of displacement vector orientation against direction of excavation

■ When relatively stiff rock mass is ahead, change of displacement vector orientation towards direction of excavation

■ Stiffness contrast and length of zone up to a critical thickness influences the magnitude of the deviation of the displacement vector orientation

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CASE HISTORY

Vertical displacements (crown settlement)

Trend displacement vector orientation

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CASE HISTORY

Fault zone

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DEVELOPMENT OF EXPERT SYSTEM FOR DATA INTERPRETATION

■ Combining several trends allows for identifying also the spatial orientation of single features or zones with fifferent properties ahead of the face

■ Existing knowledge is used to establish typical displacement trends for various geotechnical conditions

■ Establishment of a correlation matrix allows automated predition of situation ahead

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ESTABLISH TYPICAL TREND COMBINATIONS

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TREND CORRELATION MATRIX

Lenz, G. 2007. Displacement monitoring data in tunnelling. Development of a semiautomatic evaluation system. Diploma thesis TUG

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COMPARISON OF ACTUAL TRENDS TO REFERENCE TRENDS

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EVALUATION

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PREDICTION

■ Reference case 3.7 is most likely to apply.This means stiffer rock mass ahead, with vertical dip and a strike from the right to the left

Predicted situation

Grossauer, K. 2009. Expert System Development for the Evaluation and Interpretation of Displacement Monitoring Data in Tunnelling. Doctoral thesis TUG

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EVALUATION OF LINING UTILIZATION

■ Strains in the lining are evaluated from measurement results; as displacements are measured in several points, spline has to be used to assess strains between measurement points

■ For minimizing the error, distance between single monitoring points should be small

■ Appropriate material model for complex shotcrete properties should be chosen for evaluating stresses in the lining

■ Actual strength then is compared to the calculated stress

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Monitoring


EXAMPLE: circular tunnel, dia 10m; 30cm shotcrete,advance rate 2m/d

■ Development of displacements (radial symmetric)

0

10

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30

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50

60

70

80

90

0 5 10 15 20 25 30 35 40 45 50

Time [d]

Dis

plac

emen

ts [m

m]

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Monitoring


EXAMPLE: circular tunnel, dia 10m; advance 2 m/d

■ Lining utilization

0,00

0,20

0,40

0,60

0,80

1,00

0 2 4 6 8 10 12 14days

stre

ss in

tens

ity

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Monitoring


EXAMPLE: same conditions, but advance rate 4 m/d

0

10

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0 5 10 15 20 25 30 35 40 45 50

Time [d]

Dis

plac

emen

ts [m

m]

0,00

0,20

0,40

0,60

0,80

1,00

0 2 4 6 8 10 12 14days

stre

ss in

tens

ity

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EVALUATION OF LINING UTILIZATION BASED ON MONITORED DATA (TUNNEL:MONITOR)

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CONTOUR PLOT OF SURFACE DISPLACEMENT

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SUMMARY

■ For observational methods monitoring is an integral part of the final design

■ Monitoring is the key element for observing the system behaviour and predicting the ground conditions

■ Monitoring is the basis to verify the design assumptions, detect behaviour deviating from the normal, and implement remedial measures in time

■ Modern methods of measurement and data evluation allow controlled tunnel construction

■ Necessarily data have to be of good quality, and the persons using the data need to know what they mean

■ Appropriately using the available tools significantly reduces the residual risks and allows saving time and money

state of the art in tunnel monitoring - ytmkytmk.org.tr/files/files/09_tug_monitoring.pdfaustrian...

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